Support lowercase error codes in `--explain`

[Skalman]

It would be nice if rustc --explain supported lowercase error codes:

The current output is:

$ rustc --explain e0106
error: e0106 is not a valid error code

Ideally, the command would work regardless of whether you enter e0106 or E0106:

$ rustc --explain e0106

This error indicates that a lifetime is missing from a type. If it is an error
inside a function signature, the problem may be with failing to adhere to the
lifetime elision rules (see below).

[...]

[inquisitivecrystal]

@rustbot label +A-driver +A-error-codes +C-enhancement +T-compiler
@rustbot claim

[inquisitivecrystal]

I'm just about to open a pull request for this. Thank you for bringing it up!